sched/cache: Keep nr_pref_llc_running in the runnable domain, to fix LLC mis-scheduling buggrafted

alb_break_llc() decides whether to break LLC preference during active
load balance. It does so by testing that every runnable fair task on the
source rq prefers its LLC:

	env->src_rq->nr_pref_llc_running == env->src_rq->cfs.h_nr_runnable

But the two counters cover different sets. nr_pref_llc_running is updated
in account_llc_enqueue()/account_llc_dequeue(), next to cfs_rq->nr_queued,
so it follows queued tasks. h_nr_runnable is updated in set_delayed()/
clear_delayed() and drops delay-dequeued tasks.

So under DELAY_DEQUEUE, a preferring task that goes to sleep stays counted
in nr_pref_llc_running while h_nr_runnable falls. The equality then breaks,
alb_break_llc() returns false, and active balance is free to pull a task
off its preferred LLC. Active balance only moves runnable tasks, and this
is the only LLC check it consults: once the stopper runs, LBF_ACTIVE_LB
skips the per-task test in can_migrate_task(). The runnable set is the one
we want.

Fix it on the counter side. A task should be counted in
nr_pref_llc_running exactly while it is both queued on its preferred LLC
(pref_llc_queued) and runnable (!sched_delayed). Define that membership
once in task_pref_llc_runnable(), and adjust the counter only through
pref_llc_running_inc()/pref_llc_running_dec() from the four sites that
change either input: account_llc_enqueue(), account_llc_dequeue(),
set_delayed() and clear_delayed(). Gating every update on the same
predicate keeps the delay, wake and dequeue paths from double-counting
or underflowing; see the comments at those sites for the ordering.

nr_llc_running and sd->llc_counts are not touched and stay on queued
semantics.

Fixes: 714059f79ff0 ("sched/cache: Handle moving single tasks to/from their preferred LLC")
Closes: https://lore.kernel.org/lkml/20260827135000.735138-1-zhanxusheng@xiaomi.com/
Reported-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Suggested-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Kayra Cizmeci <kayracizmeci@gmail.com>
Cc: <stable@kernel.org> # v7.2.x
Link: https://patch.msgid.link/06af61afedac32e6477f57feb4d658f6c411c3af.1790035273.git.tim.c.chen@linux.intel.com

1 files changed, 36 insertions, 0 deletions

diff --git a/Documentation/gpu/nova/index.rst b/Documentation/gpu/nova/index.rst
new file mode 100644
index 000000000..2afa58e8f
--- /dev/null
+++ b/Documentation/gpu/nova/index.rst
@@ -0,0 +1,36 @@
+.. SPDX-License-Identifier: (GPL-2.0+ OR MIT)
+
+=======================
+nova NVIDIA GPU drivers
+=======================
+
+The nova driver project consists out of two separate drivers nova-core and
+nova-drm and intends to supersede the nouveau driver for NVIDIA GPUs based on
+the GPU System Processor (GSP).
+
+The following documents apply to both nova-core and nova-drm.
+
+.. toctree::
+   :titlesonly:
+
+   guidelines
+
+nova-core
+=========
+
+The nova-core driver is the core driver for NVIDIA GPUs based on GSP. nova-core,
+as the 1st level driver, provides an abstraction around the GPUs hard- and
+firmware interfaces providing a common base for 2nd level drivers, such as the
+vGPU manager VFIO driver and the nova-drm driver.
+
+.. toctree::
+   :titlesonly:
+
+   core/guidelines
+   core/todo
+   core/vbios
+   core/devinit
+   core/fsp
+   core/fwsec
+   core/falcon
+   core/tlv